Of Particular Significance

Looking for Signs of Dark Matter at the Milky Way’s Center

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/19/2012

There is going to be some amount of debate regarding dark matter in the next few weeks, so I’ve written an article on one of the best ways to go looking for new signs of dark matter out in space.

The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects — for instance, its effect on the motions of stars within galaxies, its ability to bend light a la Einstein, etc.  It’s almost certain that most of a galaxy is dark matter.  And over the years we’ve convinced ourselves this dark matter almost certainly can’t be made from any type of particle that we already know about.

But to learn more about what it is, we need to find signs of some of its non-gravitational effects, if it has any.  One possibility is that dark matter particles, if and when they collide, might annihilate into ordinary known particles.  If those known particles are photons, we might be able to detect them.  A good way to look for them would be to point a suitable telescope toward the center of the Milky Way, our galaxy, which is one place where we expect dark matter particles to be especially numerous, and collisions among them to be especially common.

In the article I just finished, I explain how this can be done.  One goes looking for photons from the galactic center, makes a plot of the number of photons observed at a particular energy, and looks for a bump in the plot — an exceptional number of photons with the same energy.

And the reason I’m doing this now is that there is a new paper claiming that a signal of this type may have been seen (with a claimed significance of 3.3 standard deviations, after including the look-elsewhere effect.)  This is a paper by a theorist, analyzing publicly available data taken by the experimental group that operates the Fermi Large Area Telescope satellite.  One should note that the record of theorists making discoveries using experimentalists’ data is very poor.  Typically there are either detector-related or statistics-related issues that theorists screw up.  And there are risks of bias — I am not yet sure whether the rather sophisticated analysis method used by this theorist was chosen in a blinded fashion.  [For instance, did he choose his method first and then look at the data, or did he already know there was a hint of a peak in the data before he started designing his method?] So I would be skeptical of this claim for now.  (And the theorist, knowing he’s out on a limb, was careful [and wise] to put the word “Tentative” in his title.)   However, stranger things have happened, so I wouldn’t dismiss this claim out of hand either, at least not until the Fermi experimentalists tell us that in their opinion the theorist over-estimated the statistical significance of this particular bump.  We’ll be looking forward to what they have to say.

I’ll have a few more details about this for you soon.

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25 Responses

  1. a nice note from Christoph Weniger

    You are right, there are not many left. Actually, since we know from observations how many are left, we can infer their self-annihilation rate in the early Universe, where it was most efficient. In most cases, they would have the same annihilation rate today, and this is why we know that there is hope to see them in cosmic-ray experiments.

    See for example http://arxiv.org/pdf/hep-ph/0404175 page 10 for details.


  2. In the world of ideas, there is always pros and cons, like the article from a few days ago…“This doesn’t mean that dark matter does not exist,” says study co-author Christian Moni Bidin, an astronomer at the Universidad de Concepcion in Chile. “The result is only that dark matter is not where we expected it.”


    as with neutrino fiasco, lets observe and measure correctly and time will tell.

  3. on dark matter annihilation – If one were to assume WIMPs are dark matter components:
    One thing that bothers me about annihilation of Weakly Interactive Massive Particles (should they exist ) via > gamma rays – if they were quasi Majorana particles and therefore it’s own antiparticle, how could there be still so many left, – or vice versa, if a decay would be better expression, they would have to have a very long half life…even proton is considered to have a half life of some ~ 10^36 years..

    1. As the universe expands, and the dark matter particles annihilate with themselves (or, if they are not Majorana particles, with their anti-particles) they eventually become so dilute that the rate at which annihilation occurs becomes completely negligible (except, a long time later, in the centers of galaxies where we are currently looking for signs of these annihilations.) The only question is: how many are left over after the universe cools down?

      The main reason people like WIMPs (massive particles that are affected by weak nuclear forces, or Weakly-Interacting Massive Particles) is that under very simple dynamical assumptions, when one calculates the number left over, it comes out just about right. But it’s not hard to come up with other models/scenarios/particles for which the number also comes out right, so I think people often make too much of this coincidence (which is sometimes jokingly called the `WIMP miracle’… doesn’t seem that miraculous to me.)

      Dark matter particles clearly have an extremely long life, and perhaps are permanently stable. But we don’t know anything other than the obvious facts that many of them are still around, and if significant numbers of them were decaying to visible particles we’d probably have noticed. Still, looking for their decays is just as worthwhile as looking for their annihilation. The technique is basically the same.

  4. Could a dark matter particle be created and detected at the LHC ?

    i.e. could a dark / anti-dark pair be produced in a collision ?

    1. Yes, dark matter particles may be produced. At a minimum, we can always look for them the way we look for any invisible particles — by looking for a jet (from a stray quark or gluon) recoiling against nothing. Neutrinos are a background to this process, so we have to measure and calculate the backgrounds very carefully to be able to detect a signal. But this is possible. I wrote about this here (see Figure 3): http://profmattstrassler.com/2012/03/05/news-from-la-thuile-with-much-more-to-come/

  5. Why is it almost certain that dark matter exists? Why is it not almost certain that the measurements are wrong or based on wrong parameters i.e. gravitaton à la Einstein is wrong? Are you sure that the dark matter solution for calculations that give “wrong” results is premature methodologically? Not checking on/reconsidering the basic starting-points may result in naturalistic fallacy, leading us further away from reaching our goals.

  6. Hi Matt:

    I´ve noticed that you are still shaken by the OPERA experiment, you mention it twice but your post is about the signs of dark matter at the Milky Way’s center. Anyway, I´m still skeptical about the issue and the measurements. OPERA-3, Borexino and MINOS+ stand by but so far we know that the neutrino is as fast as light.

    I´m also skeptical about the third law of Newton. The third law says that for every action there’s an equal and opposite reaction, therefore is not possible to propel a body outside its own dimensions. The experiments made by Eric Laithwaite, professor of electrical engineering of Imperial College in London, seem to me astonishing. Prof. Laithwaite invented the linear electric motor, a device that can power a passenger train. In the 1970s he and his colleagues combined the linear motor with the latest hovercraft technology to create a British experimental high speed train. This was a highly novel, but perfectly orthodox technology.

    The advantages of such a tracked hovercraft are obvious to anyone who sees a hover-rail train running along, suspended in the air above the track — it is quiet, has no moving parts to wear out and is practically maintenance-free. The significance of this last point quickly becomes clear when you learn that more than 80 per cent of the annual running costs of any railway system is spent on maintenance of track and rolling stock because of daily wear.

    Laithwaite also investigated the properties of the gyroscope spinning and observed that the gyroscope appeared to be producing a force without a reaction, thus defying the third law of Newton. Similar phenomenon happened with the gyroscopes at the Gravity Probe B, launched 20 April 2004, a space experiment testing two fundamental predictions of Einstein’s theory of General Relativity (GR), the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Data collection started 28 August 2004 and ended 14 August 2005. Analysis of the data from all four gyroscopes results in a geodetic drift rate of -6,601.8±18.3 mas/yr and a frame-dragging drift rate of -37:2±7.2 mas/yr, to be compared with the GR predictions of -6,606.1 mas/yr and -39.2 mas/yr, respectively (‘mas’ is milliarc-second; 1 mas= 4.848 X10-9 radians or 2.778 X10-7 degrees).

    Not all the scientists agree with the results, they claim that some electrostatic anomalies that affected the gyroscopes weakened the experiment. In short, we move on a fragile line whether the measurements and the physical laws have to be observed with certain dosage of scepticism.


  7. I really appreciate your blog Dr. Strassler, and I come here every time I read an interesting bit of particle physics news (you often have comments on the latest breakthroughs). When I saw the title of this post, I assumed it was related to the news story of the missing dark matter:


    Do you have any thoughts on that?

    However, I mainly came here to see if you had anything to say about the news of electron split into quasi-particles. It’s been widely reported, but there seems to be confusion over the significance of the discovery, or even its exact meaning. I always wait for your input before getting too excited. 🙂


    1. I’m looking into the dark matter thing right now. It’s certainly an interesting claim; the paper is too complex for me to read it quickly, so it may be a few days before I understand it. That said, remember most radical claims turn out to be wrong, or at least only partially right. So I wouldn’t take this result (or any claim like this, in any scientific field) too seriously until it’s been verified by other groups of people.

      As for electrons “splitting” inside of metals, this kind of thing is very misleading if you don’t explain it properly. It’s a well-known phenomenon, but it isn’t what you think. I’d need a whole article to explain it, maybe several. But don’t get excited; it’s nothing unexpected, and nothing as dramatic as it sounds.

  8. Matt, you say “The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects”
    I have encountered some people questioning the “scientificity” of Dark Matter (hypothesis?) such as lack of alternative models, lack of empirical evidence beyound its gravitational effects (that could be explaining simply modifiying the theory of gravity for long distances, according to them) or even lack of a way to proof Lambda-CDM wrong.
    Listening to someone in the field is very clarifying. What’s your position about this?

    1. Most people who question this stuff have never actually looked at the extensive data. It is by no means dogma; people in the scientific community have questioned it for as long as I have been in the field. But the dark matter hypothesis has passed test after test after test; time after time I’ve seen people think the hypothesis was about to break down due to a new observation, but then it turned out the observation was wrong.

      The reason there are no (or, more precisely, very few) alternative models is because all the other ones were killed off by data! There used to be lots more of them. People are running out of alternative ideas because the dark matter hypothesis fits the data so well.

      As for lack of evidence beyond gravitational effects, we’re looking for non-gravitational effects in many different ways; however it is possible that dark matter has NO measurable non-gravitational effects, so we can’t exclude the dark matter hypothesis if we don’t see any. Of course if we find such effects, that will be great!

      To suggest Lambda-CDM couldn’t have been proven wrong is ridiculous! The WMAP satellite’s results could easily have killed off Lambda-CDM. In fact, one of the main goals of WMAP was to test Lambda-CDM. But rather than killing it off, it found excellent agreement with it.

      We’ll keep testing. But so far, the dark matter hypothesis works very well. Would I be shocked if it turned out to be wrong? If you’d asked me that 20 years ago, I would have said “no”. Today, yes, I would be a little shocked. (But not as shocked as I would have been had it turned out that neutrinos can travel faster than light.)

        1. There are many assumptions that go into the technique. I cannot yet evaluate whether I believe the result; I’m probably not expert enough to point out its flaws, it it has any. I’m asking around.

          Suffice it to say that you should remember the OPERA experiment and not believe everything you read, at least not until it has been cross-checked by other independent researchers.

    2. Matt, if I could expand on your comment, identifying the gravitational effects with new things have a precedence in the discovery of Neptune so it doesn’t look that bad. As for the alternatives like f(R) they sacrifice the nice interpretation of Eintein’s equations and say with must set a number of coefficients by the data, but is obvious that with enough parameters we can fit anything, so I don’t thing modifying GR is a much better idea than Dark Matter.


      1. Well, I can’t help playing devil’s advocate and pointing out that modifying gravity has its own precedent, as you will agree; some tried to explain the shifting perihelion of Mercury through a new planet before Einstein’s gravity theory came out. So the history of the solar system gives us examples that tell us that either is possible.

        The real trick in the case of Neptune was that people actually discovered Neptune and settled the issue. I hope we’ll be able to discover dark matter directly too, but there is no guarantee, unfortunately, that a direct detection will ever be possible.

        Meanwhile, if someone can propose a consistent theory that really fits all the data and is really is a sensible alternative to dark matter, that would be great. (It does have to be sensible, however.) It’s always best, when trying to make sure your understanding is complete, to have two alternative explanations which both agree well with current data but give differing predictions elsewhere that future experiments can try to distinguish.

      2. Yes you’re completely right that in the case of Neptune somedoby actually found the planet and so we must hope that Dark Matter is found by others non-gravitational means before declaring it the right answer.

        And surely it’s better to have multiple hypothesis that fit data and let the future sort it out, I’m not paricularly fond of DM, but it seems as good as anything else in the market.

        By the way, the perihelion shift did not led to modfiyng gravity, Einstein did it because of equivalence principle, the perihelion shift was just a lucky consequence. But I get your point, although the precision tests of GR are very restrictive the last time I checked.

        Thanks Matt for the discussion, it is always very illuminating

  9. Ken—there are no implications. The Higgs lifetime is less than a picopicosecond and can’t be dark matter. Yes, two numbers are similar, but the masses of the tau and charm are similar, but they aren’t related either.

    1. What Marc (professor at William and Mary) says is correct, of course. Dark matter can’t actually be the Higgs particle, certainly. But let me elaborate further.

      When you see two numbers that are similar, you can always ask: are they related, or are they coincidence? History teaches that the answer can be either one.

      The neutron and proton masses are very similar. That turns out to be no accident — they are deeply related.

      But (as Marc essentially said) the tau lepton has a mass of 1.776 GeV and the lightest hadron containing a charm quark is 1.864 GeV. Although these are similar, it appears they are completely unrelated; if anything, the tau lepton’s mass is more likely related to that of the bottom quark, from which it differs by quite a lot, through grand unification of the three non-gravitational forces.

      Another example: if you take the average of the W and Z masses, you get 86 GeV. Twice that is 172 GeV, very close to the top quark mass. Is this somehow important? Almost certainly not; these masses come from quite different places.

      There’s another point. Look at the claimed result from this new dark matter paper, which suggests a dark matter particle of mass 129.8 +=2.4{+7}{-13} GeV. In other words the one-standard-deviation band is 20 GeV wide, which means that this particle could be roughly anywhere between 100 and 150 GeV. Now the data is only being examined in the range from 20 – 300 GeV. In that range we have the W and Z particles, the Higgs and the top quark. So the chance that the biggest bump in the Fermi photon spectrum would overlap with the mass of a known particle isn’t very small. And would you also have gotten excited if the mass had been twice as big or half as big as the W or Z or Higgs or top? Because then the chances of a coincidence would have been almost 100%. Coincidences are very common.

      Finally, the Higgs mass gets large quantum corrections; it’s not something that we just write down in the equations on the first line. We have to calculate it in any given model and the calculation often is not simple. For instance, in the simplest supersymmetric models, the Higgs mass before quantum corrections has to be below 90 GeV; only through large quantum corrections can it get up into the 115-130 GeV range. We have no idea where the dark matter particle’s mass might be coming from since we have no idea what kind of particle it is. So there’s no way, right now, to relate these two quantities from the theoretical point of view. Maybe we’ll see some attempts to link the two in the coming weeks.

  10. Why did you not mention that the energy of the possible dark matter observation overlaps that of the possible Higgs LHC signal? What are the possible implications of that?

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